Since I haven’t found the solution in grammar books, I’ve just guessed it was the same thing after genitive singular masculine nouns as after nominative singular feminine nouns since they behave in the same way in most points of view. But maybe I’m wrong.

You already cited the rules: lenition after masculine genitives ending in slender consonant only:
So, in an tseomra suí, suí is unlenited because seomra ends in -a.

A problem is an mhic tíre. You could expect lenition but there is none. (at least in examples in Nua-Chorpas or logainmneacha: Clais an Mhic Tíre)
Perhaps, mac tíre is too lexicalized a noun, so though there’s (sometimes) declension of mac, tíre is unchanged.